1. Field of the Invention
This invention relates a copper-based alloy that contains no environment-unfriendly Pb, has excellent corrosion resistance against dezincing corrosion even when used in the presence of a corrosive aqueous solution, and is excellent in hot workability and cuttability.
2. Background Art
Among copper-based alloys, the Cu—Zn alloys (brasses) have long been widely used for plumbing equipment, gas valves and the like because of their excellent hot workability, cuttability and the like. Known types include brass for forging (JIS C 3771), free-cutting brass (JIS C 3604) and high-strength brass (JIS C 6782), but all of these copper-based alloys contain large amounts of lead, which is known to be unfriendly to the environment.
These alloys are also characterized by very inferior dezincing resistance, particularly in an environment where corrosive aqueous solution is present, because of its preferential elution owing the strong ionization tendency of Zn in the β phase.
Various methods have been proposed for improving the dezincing resistance of brass containing lead. For example, Patent Reference 1 listed below teaches improvement of dezincing resistance by adding Sn to Cu—Zn alloy and further controlling γ phase ratio and Sn concentration in the γ phase through various heat treatments after hot extrusion, and Patent Reference 2 teaches improvement of dezincing resistance by adding Sn and Si at certain rates to Cu—Zn alloy containing Pb.
Proposals have also been made regarding obtaining a suitable degree of cuttability in free-cutting brass without Pb (with no added Pb). Patent Reference 3, for instance, teaches that cuttability and strength can be enhanced by adding Si to Cu—Zn brass and Patent Reference 4 discloses a brass-bismuth alloy containing not more than 0.1% Pb that maintains the cuttability of free-cutting brass thanks to addition of Bi. Patent Reference 5 discloses a lead-free free-cutting brass that is leadless and good in dezincing resistance.
Patent Reference 1 JP-A-Hei-10(1998)-183275
Patent Reference 2 JP-A-2002-12927
Patent Reference 3 JP-A-2000-119774
Patent Reference 4 JP-A-Sho-54(1979)-135618
Patent Reference 5 JP-A-2002-3967
As taught by the above Patent References, a leadless free-cutting brass can retain free-cuttability by addition of Bi or Si and, in fact, leadless free-cutting brasses available on the market are divided mainly into two types: Bi system and Si system. However, since leadless brasses use Pb-free starting materials, Bi-master alloy, Cu—Si-master alloy or the like, the cost of the starting materials is inevitably high. Another problem is that the fact that the brasses are divided into to two types complicates scrap sorting and distribution.
Moreover, the leadless free-cutting brass of Patent Reference 5 does not readily exhibit dezincing resistance effect unless the β phase, which is poor in dezincing resistance, is fragmented. As this makes the β phase area small at high temperature, the hot workability becomes inferior. In addition, the dezincing resistance property varies depending on the form and area ratio of the α and β phases, making it necessary to conduct heat treatment after hot working or extrusion. The leadless free-cutting brass is therefore not readily applicable to forged and other products that are not heat-treated after hot working.
An object of the present invention is to overcome the foregoing problems by providing a copper-based alloy excellent in dezincing resistance that achieves improved dezincing resistance, hot forgeability and cuttability without containing Pb and can be produced at low cost.